Device for maintaining a tool attachment in a knuckle boom crane on a constant level above the ground

ABSTRACT

The invention relates to a device for maintaining a tool attachment in a knuckle boom crane, preferably for forest machines, on a constant level above the ground. The knuckle boom comprises a main boom which can be raised and lowered by means of a double-acting hydraulic cylinder (4), and an outer boom pivotal relative to the main boom my means of a likewise double-acting hydraulic cylinder (6). To ensure the vertical adjustment of the tool attachment, the lift cylinder (4) is connectible to the outer boom cylinder (6) via a compensator (21) in such a manner that an amount of hydraulic fluid which is proportional to the amount of hydraulic fluid which upon activation of the outer boom is pressed out from the lifting or the lowering side, respectively, of one of the cylinders (4, 6), is pressed into the lowering or the lifting side, respectively, of the other cylinder. The compensator comprises at least one hydraulic cylinder (22) having two chamber (23, 26) which are separated by a piston (31) movable in the cylinder and having two different working areas or displacements (24, 25) in the two chambers (23, 26). The lift cylidner (4) is connectible to one chamber (26) and the outer boom cylinder to the other chamber (23).

TECHNICAL FIELD OF THE INVENTION

This invention relates to a device for maintaining a tool attachment ina knuckle boom crane, preferably for forest machines, on a constantlevel above the ground, the crane comprising a main boom which can beraised and lowered by means of a first double-acting hydraulic cylinder,and an outer boom pivotal relative to the main arm by means of a seconddouble-acting hydraulic cylinder.

BACKGROUND OF THE INVENTION

Knuckle boom cranes have been extensively used because of theircapability of reaching any desired point within the volume the toolattachment or the load application point describes. This is achieved byseparate operation of the crane post, main boom, outer boom andtelescopic part of the outer boom. Even in normal use, concurrentoperation of all these functions requires an experienced, clear-thinkingoperator.

It has proved desirable to employ vehicle-borne knuckle boom cranes ofthe above-mentioned type also in mechanized tree felling and primarilyin shrub cleaning, the felling or cleaning tool being attached at theload application point of the outer boom, since special machines cannotbe used for economic reasons. Shrub cleaning means that a hydraulicallydriven flail is swung back and forth at a constant vertical level, bothperpendicularly out from and along the direction of travel of thetractor. The vertical position of the flail above the ground should beapproximately constant. This is achieved by constant changes of themutual angular positions of the main boom and the outer boom, often incombination with telescopic extension of the outer boom. The cleaningoperation must be effected rapidly since large areas are to be covered,and the suitable cleaning period is short. This means that the drivermust run the vehicle and operate the outer boom while ascertaining thatvaluable plants and trees are spared and that the flail gets clear ofstones and of the ground. This has proved troublesome with outer boomsof the standard type. To facilitate this work, it is desirable to have alinear horizontal tool path which can be controlled by the driver with asingle control instead of three. Such a path is obtained if the oilflows to the hydraulic cylinders of the main boom and the outer boom arepositively controlled in such a manner that if the tip of the main boommoves upwards a predetermined vertical distance, the tool attachment ofthe outer boom is lowered the same distance. The tool thus remains onthe same vertical level while simultaneously moving inwards towards thevehicle. Such a function is in principle achieved if the hydraulic fluidconduits of the outer boom are so connected that the oil which ispressed out from the lift cylinder when the main boom is raised, insteadof returning to the oil tank, is conducted to the lowering side of theouter boom cylinder. However, this presupposes that the geometry and thecylinder dimensions agree with each other under certain givenconditions. These are however determined primarily by the basic tasks tobe performed by the outer boom, such as lifting timber. Only theoreticalconditions would make it possible to use the outer boom both for timbertransport and for shrub cleaning solely by switching the hydraulic fluidconduits.

BRIEF ACCOUNT OF THE INVENTIVE CONCEPT

The present invention aims at providing a device which permits usingstandard-type knuckle boom cranes which are not primarily intended forshrub cleaning, also usable for this purpose. According to theinvention, this is achieved in that said first cylinder or liftcylinder, when desired, is connectible via at least one compensator tothe second cylinder or outer boom cylinder in such a manner that anamount of hydraulic fluid which is proportional to the amount of fluidwhich upon activation of the outer boom is pressed out from the liftingor the lowering side, respectively, of one of said cylinders, is pressedinto the lowering or the lifting side, respectively, of the other ofsaid cylinders, said compensator comprising at least one hydrauliccylinder having two pressure chambers separated by a movable pistonhaving two different working areas or displacements in said twochambers, and that the lift cylinder is connectible to one chamber andthe outer boom cylinder is connectible to the other chamber, the workingareas of said piston being so selected that the resulting hydraulicfluid flow between the lift cylinder and the outer boom cylinderpositively ensures the desired vertical adjustment of the outermostpoint of the outer boom.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

In the drawings,

FIG. 1 is a side view of a forest machine provided with a knuckle boomcrane of the type concerned,

FIG. 2 is a hydraulic flow diagram showing how the compensator isconnected between the outer boom cylinder and the lift cylinder, and

FIG. 3 is a side view on a larger scale illustrating how the loadapplication point can move after the compensator has been connected.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

In FIG. 1, a forest machine in the form of a forwarder, generallydesignated 1, carries a crane 2 of the knuckle boom type. The crane 2comprises a main boom 3 which can be raised and lowered by pivotingrelative to a crane post by means of a first double-acting hydrauliccylinder 4, and an outer boom 5 which is pivotal relative to the mainboom by means of a second double-acting hydraulic cylinder 6. The outerboom 5 can be telescopically extended by means of a double-actinghydraulic cylinder (not shown). The broken line 7 in FIG. 1 shows thetotal working range of the crane. The load application point 36 of theouter boom is provided with a tool attachment 37.

In FIG. 2, it is shown how the lift cylinder 4 for the main boom 3 isconnected by conduits 9, 10 and a directional valve 11 to a hydraulicpump 12 and an oil tank 13. The outer boom cylinder 6 is connected byconduits 15, 16 and a directional valve 17 to a hydraulic pump 18 and anoil tank 19. In normal cases, the pumps 12 and 18 constitute a singlepump, and the oil tanks 13 and 19 constitute a single oil tank. Also,the directional valves 11 and 17 may be devised as a single unit, butfor greater clarity these valves are illustrated as parts of separatecircuits. The lifting side 8 of the outer boom cylinder 6 is connectedby a conduit 20, a compensator 21 and an electrically operated valve 28to the lowering side 34 of the lift cylinder 4. The valve 28 cooperateswith an electrically operated valve 29 connected in the conduit 15 insuch a manner that when the valve 28 is closed, the valve 29 ismaintained open, and vice versa. The directional valve 17 is connectedby the conduit 15 and a conduit 30 to the lifting side 35 of the liftcylinder 4. In a preferred embodiment, the compensator consists of aconventional hydraulic cylinder 22 having a piston rod 27 and a piston31 separating two different chambers 23 and 26. The cylinder 22 is ofthe double-acting type, and the piston 31 has two different workingareas 25 and 24 because of the piston rod 27 arranged centrally on theworking area 24. In the illustrated embodiment, the valve 28 is closedand the valve 29 is open, which means that the hydraulic system isconnected for vertical positioning, here entirely controlled by anoperating lever for the outer boom cylinder 6. The full arrows indicatethe flow in the system when the load application point 36 of the outerboom 5 moves out from the vehicle while the broken arrows indicate theflow in the system when the same point moves inwards towards the vehicle

Thus, when the load application point or the outer boom tip 36 should bemaintained on the desired level and move out from the vehicle, thedirectional valve 17 is set in such a position that hydraulic fluid fromthe pump 18 is pressed to the lowering side 14 of the outer boomcylinder 6 via the conduit 16 as indicated by the full arrow. Hydraulicfluid is then pressed out from the lifting side 8 of the cylinder 6 andinto the chamber 23 of the compensating cylinder 22. As a result,hydraulic fluid will be pressed out from the second chamber 26 of thecompensator through the conduit 20 and the valve 28 and to the loweringside 34 of the lift cylinder 4. Hydraulic fluid is then pressed out fromthe lifting side 35 of the cylinder 4, through the conduit 30 and thedirectional valve 17 and to the tank 19. If the tip 36 should be movedinwards towards the vehicle, the directional valve 17 is switched suchthat the hydraulic pump 18 pumps hydraulic fluid into the lifting side35 of the cylinder 4 through the conduit 30, which means that hydraulicfluid is pressed out from the lowering side 34 of the cylinder 4,through the conduit 20 and into the chamber 26 of the compensatingcylinder 22, whereby an amount of liquid which depends upon the workingareas in the cylinder, is pressed out from the other chamber 23 and intothe lifting side 8 of the outer boom cylinder 6. Hydraulic fluid ispressed out from the lowering side 14 of the cylinder 6, through theconduit 16 and the directional valve 17 and to the tank 19. The twoabove-mentioned positions of the directional valve correspond to thelifting and the lowering position, respectively, of the valve duringnormal operation of the outer boom.

The connection of the conduit 20 between the lift cylinder 4 and theouter boom cylinder 6 means that the lift cylinder 4 will move independence upon the movements of the outer boom cylinder 6. Now, if thecompensating cylinder 22 is a standard cylinder having suitable workingareas 24, 25, the proportions between the hydraulic fluid flow enteringthe cylinder 22 and the hydraulic fluid flow leaving the cylinder willbe such that upon actuation of either the lift cylinder or the outerboom cylinder by means of the directional valve 17, the load applicationpoint of the outer boom will move in one and the same horizontal plane.By means of the directional valve 11 and the conduits 9, 10, the liftcylinder can be temporarily actuated during said vertical positioningprocedure in order to make the tool avoid such obstacles as stones orstumps. Further, this hydraulic circuit makes it possible to set thevertical position of the load application point. By means of a thusconnected hydraulic system in a knuckle boom crane, it is thus possibleto have the load application point of the outer boom move in a singleplane by means of a single control means.

In FIG. 3, it is shown how the tip 40 of a tool attachment 37 mounted inthe load application point 36 of the outer boom 5 can move (along theline 38) under the influence of a compensator in the form of a standardcylinder connected in the above-mentioned way. The line 39 hereindicates a horizontal plane. In this context, it should be pointed outthat the movement of the flail need not be exactly linear to ensureacceptable cleaning but, as shown in FIG. 3, may deviate from the idealstraight line. This means that there is a certain liberty of choice whenselecting a suitable standard cylinder for the compensator.

The present invention thus makes it possible, in an extremely simple,cost-effective and efficient manner, to equip or subsequently supplementcranes of the knuckle boom type so as to make them suited, in additionto their regular tasks, also to perform shrub cleaning The dimensioningand choice of a suitable standard cylinder can be made rapidly androutinely by means of a simple-program desk computer. If it is difficultto find a standard cylinder of suitable dimensions, there are vastpossibilities of connecting different standard cylinders in seriesand/or in parallel. For instance, it is possible to connect two or moresimilar hydraulic cylinders in parallel or, alternatively, to connecttwo or more similar and/or different cylinders in series to obtain thedesired proportions of the working areas in the compensator. It is alsopossible to connect hydraulic cylinders of different dimensions inparallel, but in such a case, their piston rods must be mechanicallylocked to each other in order to obtain a uniform movement. Themodifications to be carried out in existing hydraulic systems are small.In principle, it suffices to connect a conduit containing anelectrically operated hydraulic valve and a hydraulic cylinder betweentwo existing conduits (conduits 9 and 15 in FIG. 2), a conduit 30between two existing conduits (conduits 10 and 15 in FIG. 2), and anelectrically operated hydraulic valve 29, cooperating with the valve 28,in the conduit 15.

POSSIBLE MODIFICATIONS OF THE INVENTION

The invention is of course not restricted only to the embodimentdescribed above and illustrated in the drawings. Thus, for instance, itis also possible by means of a further compensator to adjust atelescoping movement of the outer boom to the raising and lowering ofthe outer boom and of the first boom section, respectively, so that alsothis telescoping movement will assist in maintaining the loadapplication point on the desired level. This can be achieved in that thecylinder device controlling the telescoping movement, via a furthercompensator of the abovementioned type, can be connected to the liftcylinder and/or the outer boom cylinder, suitably in series. Further,the lowering side of the outer boom cylinder can be connected to thelifting side of the lift cylinder via a suitably oriented compensatorinstead of the manner illustrated in FIG. 2. It is of course alsopossible to control the movement of the load application point with theaid of the control means of the lift cylinder instead of the controlmeans of the outer boom as described above, even though the latteralternative is preferred for practical reasons. Of course, it ispossible to obtain a completely linear movement by exact dimensioningand, optionally, special manufacture of the compensator.

I claim:
 1. A device for maintaining a tool attachment in a knuckle boomcrane, as used in forest machines, at a constant level above the ground,in which the crane comprises a main boom which can be raised and loweredby means of a first double-acting hydraulic cylinder, and an outer boompivotal relative to said main boom by means of a second double-actinghydraulic cylinder, characterized by at least one compensating hydraulicdouble-acting cylinder; connecting means connecting said compensatingcylinder between said first and second hydraulic cylinders; and valvemeans for permitting hydraulic flow between said first and secondhydraulic cylinders via said compensating hydraulic cylinder; saidconnecting means being such that an amount of hydraulic fluid which isproportional to the amount of fluid which upon activation of the outerboom is pressed out from the lifting or the lowering side, respectively,of one of said cylinders, is pressed into the lowering or the liftingside, respectively, of the other of said cylinders said compensatingcylinder having two pressure chambers separated by a movable pistonhaving two different working areas or displacements in said twochambers, and that the lift cylinder is connectible to one chamber andthe outer boom cylinder is connectible to the other chamber, the workingareas of said piston being so selected that the resulting hydraulicfluid flow between the lift cylinder and the outer boom cylinderpositively ensures the desired vertical adjustment of the outermostpoint of the outer boom.
 2. Device as claimed in claim 1, in which theouter boom can be extended by means of a double-acting hydrauliccylinder, characterized in that also said cylinder is connectible via acompensating cylinder to the lift cylinder the outer boom cylinder. 3.Device as claimed in claim 1, characterized by two or more compensatinghydraulic cylinders connected in series after each other.
 4. Device asclaimed in claim 1, characterized by two or more compensating hydrauliccylinders connected in parallel with each other.
 5. Device as claimed inclaim 1, characterized by two or more compensating hydraulic cylindersat least one of which differs from the others, said cylinders beingconnected in parallel with each other and their piston rods beingmechanically locked to each other.
 6. Device as claimed in claim 1, inwhich the outer boom can be extended by means of a further double-actinghydraulic cylinder, characterized in that said further cylinder isconnectible, via a compensating cylinder to the lift cylinder or theouter boom cylinder.